Preheat oven to 350. Butter a ceramic baking dish. Bring 4 quarts of water to boil in a large saucepan. Add Dumplings & allow water to return to a boil, lower heat & simmer for 20 minutes. Drain. In saute pan, heat oil & cook onions 2-3 minutes or until tender. Add mushrooms & continue to cook until mushrooms are golden. Remove from heat. In a large mixing bowl, combine all ingredients & toss gently to cover all with sauce. Spoon into prepared dish & bake 30-35 minutes or until heated thoroughly.